Evidence has been accumulating that cell membrane glycosphingolipids may serve as receptors for several biologically active agents including glycoprotein hormones, lymphokines and interferon and may play a role in cell activation. Using purified antibodies to aGM1, it has been demonstrated that this glycosphingolipid is expressed predominantly by cells in the T-lymphocyte lineage. The expression of aGM1 seems to be associated with functionally responsive status within the lineage. The aGM1 is expressed throughout the lineage and is exposed, masked, and re-exposed at certain stages of maturation and activation. We have demonstrated that a proportion of the multipotential hematopoietic stem cells (CFU-S) also expresses aGM1. The aGM1-positive CFU-S display a reduced self-renewal potential and are depleted in athymic mice. In the context of current theories of hematopoiesis, decreased self-renewal is associated with increased cell cycle activity and propensity to differentiate into committed progenitors. Thus the aGM1-positive CFU-S may represent mobilized stem cells, a subset of thymic dependent stem cells, or both. To attempt to resolve these questions, we propose to determine if aGM1 phenotype is associated with cell cycle activity of CFU-S and to determine if there is a difference in T cell dependency for self-renewal and differentiation of stem cell subpopulations defined by aGM1 phenotype. These studies will elucidate the bases for hematopoietic commitment and regulation at the stem cell level and thus provide a basis for studies on the mechanism of hematopoietic dysfunction in congenital, age-related and transplantation-related syndromes.